1. Field of the Invention
The present invention relates to an image processing apparatus and method for supporting imaging of an eye and, more particularly, to an image processing apparatus and method suitable for processing the tomogram of an eye.
2. Description of the Related Art
Ophthalmic examinations are prevalently made aiming at early diagnosis of lifestyle diseases or high-ranking diseases that lead to blindness. An ophthalmic tomography imaging apparatus such as an OCT (Optical Coherence Tomography) apparatus is useful for diagnosis of a disease because it allows three-dimensionally observing the internal state of the retina layer.
FIG. 14 is a schematic view showing tomograms of the macular portion of retina captured by the OCT apparatus. The OCT apparatus obtains three-dimensional image data formed from a plurality of tomograms, as shown in FIG. 14. Referring to FIG. 14, T1 to Tn are two-dimensional tomograms of the macular portion. In the tomogram Tn, L1 is the boundary (to be referred to as an ILM boundary) between the inner limiting membrane and the overlying tissue. L2 is the boundary (to be referred to as an NFL boundary) between the nerve fiber layer and the underlying layer. L2′ is the nerve fiber layer (to be referred to as NFL). L3 is the boundary (to be referred to as an IPL boundary) between the inner plexiform layer and the underlying layer. L4 is the boundary (to be referred to as an OPL boundary) between the outer plexiform layer and the underlying layer. L5 is the boundary (to be referred to as an IS/OS boundary) between the junction between the inner and outer segments of the photoreceptor cell and the overlying layer. L6 is the boundary (to be referred to as an RPE boundary) between the retinal pigment epithelium and the underlying layer.
Making an image diagnosis using the OCT apparatus requires a technique for specifying the boundary of each retina layer by image analysis. For example, if the ILM boundary L1 and NFL boundary L2 in FIG. 14 can be specified to measure the thickness of the NFL, the thickness can be used as one index for glaucoma diagnosis.
Various retina-layer, boundary-specifying algorithms have been proposed up to now. As a problem common to them, it becomes difficult to specify a retina-layer boundary upon a change of the layer structure owing to an artifact or lesion. For example, if a blood vessel or lesion exists in the retina layer, the luminance at a position deeper than the blood vessel or lesion drops or the retina layer locally swells. Only a single algorithm hardly specifies a retina-layer boundary. FIG. 15A exemplifies a tomogram in which a blood vessel V1 exists. FIG. 15B exemplifies a tomogram in which a hemorrhage B1 exists. FIG. 15C exemplifies a tomogram in which “detachment” of a vitreous cortex H1 and “cyst C1” exist. When the blood vessel V1 or hemorrhage B1 exists as shown in FIG. 15A or 15B, the luminance below it decreases, making it hard to see the boundary. In some cases, a lesion appears inside and outside the retina layer, as shown in FIG. 15C. In this case, no luminance decreases, but the entire retina layer swells or the originally existent retina-layer boundary is disconnected. When analyzing such a tomogram, an area where the layer structure has changed needs to be specified to switch processing to one optimum for changing the layer structure.
In Japanese Patent Laid-Open No. 2009-066015 (to be referred to as literature 1), statistical feature amounts above and below a pixel of interest are calculated in each A-scan, and it is determined that an artifact exists in an A-scan whose feature amount is equal to or smaller than a threshold. In Japanese Patent Laid-Open No. 2007-325831 (to be referred to as literature 2), a fundus image and tomogram are aligned, a blood vessel area extracted from the fundus image is projected onto the tomogram to specify the blood vessel position, and the blood vessel area within the tomogram undergoes different processing.
However, the conventional techniques have the following problems. The artifact specifying method in literature 1 considers that all artifacts arise from a blood vessel. This method does not discriminate an artifact caused by the blood vessel V1 as in FIG. 15A from one caused by the hemorrhage B1 as in FIG. 15B. Considering processing of interpolating a boundary within the artifact area, the same processing cannot be applied to an artifact caused by a blood vessel and an artifact caused by a hemorrhage because the range of the artifact area and the degree of change of the layer structure differ between these artifacts. The method in literature 2 switches processing by projecting, onto a tomogram, a blood vessel area extracted from a fundus image. Needless to say, this cannot be implemented unless a fundus image of the same patient as that of the tomogram is used. Further, this method cannot deal with a lesion which cannot be detected from a fundus image. There are lesions which hardly appear in a fundus image, such as detachment of the vitreous cortex H1 and the cyst C1 as shown in FIG. 15C. Hence, this method cannot cope with a lesion or the like which can be detected only from a tomogram.